1. Field of the Invention
This invention relates to a traction control system for motor vehicles, and more particularly it pertains to improvements in such a system adapted for a road surface having an extremely low friction coefficient or an uphill road surface having a low friction coefficient.
2. Description of the Prior Art
Known in prior art there is a traction control system for motor vehicles, which is utilized, when the motor vehicle runs on a low-friction road surface such as a frozen, snow-covered or muddy one, to prevent a slipping of the driven wheel, likely to occur when attempting to start or accelerate a motor vehicle, thereby enhancing the starting/accelerating performance as well as the running stability of the vehicle. In such a control system, the driven and non-driven wheel speeds are compared with each other, and when the former becomes higher than the latter by a predetermined amount, the driven wheel is judged as slipping, and thereupon the opening of the engine throttle is reduced to decrease the engine output so that the slip of the driven wheel is restrained, as disclosed in British Patent Publication No. 2,130,757, for example.
More specifically, with the foregoing traction control system, a throttle opening/closing control is effected by using, as control threshold value for the driven wheel speed, a value Vs provided by adding a predetermined value Vk to non-driven wheel speed Vv which is equal to the vehicle speed, as shown in FIG. 1, for example. In FIG. 1, when the accelerator pedal is depressed at time to the throttle opening degree is excessive, the driven wheel is caused to slip and thus traction control is commenced at time t2 when the driven wheel speed exceeds the above-mentioned control threshold value Vs, so that the throttle opening degree is decreased; and thereafter, the throttle opening/closing control is performed such that the driven wheel speed Vr approaches the control threshold value Vs through comparison of the driven wheel speed Vr and the control threshold value Vs. Thus, the throttle closing control is effected at a predetermined closing speed whenever the driven wheel speed Vr exceeds the control threshold value Vs, whereas the throttle opening control is effected at a predetermined opening speed whenever the driven wheel speed reaches a high peak where it changes from an increasing value to a decreasing value.
However, the foregoing conventional traction control system has the following drawbacks in that even when the motor vehicle starts on a frozen or snow-covered road surface having an extremely low friction coefficient (hereafter; an "extremely low-.mu." road surface) or an uphill road surface having a low friction coefficient (hereafter a "low-.mu." road surface) the above-described type of traction control is effected in a control mode similar to that used when the motor vehicle starts on a normal road surface instead of a extremely low-.mu. road surface or a low-.mu. uphill road surface so that control threshold value for the driven wheel speed Vr and the opening/closing speed for the throttle opening/closing control for the cases where the motor vehicle starts on an extremely low-.mu. road surface or a low-.mu. uphill road surface, are set up to be those suitable for the cases where the motor vehicle starts on a normal road surface. Thus when the throttle command value is too greatly and sharply increased or decreased an excessive slip of the driven wheel speed Vr could occur in frequent repetition; and it becomes difficult to rapidly constrain the driven wheel in such an optimum slipping state that the driven wheel speed Vr properly exceeds the non-driven wheel speed Vv; consequently, a long time is taken before the non-driven wheel speed Vv occurs, and even if the traction control is commenced at the time t2, the motor vehicle is unable to move until time t4.